The continuous trend in the development of electronic devices has been to minimize the sizes of the devices and to improve functionalities of the devices. While the current generation of commercial microelectronics are based on sub-micron design rules, significant research and development efforts are directed towards exploring devices on the nano-scale, with the dimensions of the devices often measured in nanometers or tens of nanometers. In addition to the significant reduction of individual device size and much higher packing density as compared to microscale devices, nanoscale devices may also provide new functionalities due to physical phenomena on the nanoscale that are not observed on the micron scale.
For instance, electronic switching in nanoscale devices using titanium oxide as the switching material has recently been reported. The resistive switching behavior of such a device has been linked to the memristor circuit element theory originally predicted in 1971 by L. O. Chua. The discovery of the memristive behavior in the nanoscale switch has generated significant interest, and there are substantial on-going research efforts to further develop such nanoscale switches and to implement them in various applications. One of the many important potential applications is to use such a switching device as a memory unit to store digital data.
Memristor switch devices, which are often formed of nanoscale metal/metal oxide/metal layers, employ an “electroforming” process to enable resistive switching. The electroforming process involves a one-time application of a relatively high voltage or current that produces a significant change of electronic conductivity through the metal oxide layer. The electrical switching arises from the coupled motion of electrons and ions within the oxide material. For example, during the electroforming process, oxygen vacancies may be created and drift towards the cathode, forming localized conducting channels in the oxide. Simultaneously, O2− ions drift towards the anode where they evolve O2 gas and cause physical deformation of the junction. The gas eruption often results in physical deformation of the oxide (e.g. bubbles) near the locations where the conducting channels form and delamination between the oxide and the electrode. The conducting channels formed through the electroforming process often have a wide variance of properties depending on how the electroforming process occurred. This variance of properties has relatively limited the adoption of metal oxide switches in computing devices.
In addition, in order to be competitive with CMOS FLASH memories, the emerging resistive switches need to have a switching endurance that exceeds at least millions of switching cycles. Reliable switching channels inside the device may significantly improve the endurance of these switches.